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Comparison Simulation between Ventilation and Recirculation of Solar Desiccant
Cooling System by TRNSYS in Hot and Humid Area
MMS DEZFOULI, SOHIF MAT, K.SOPIAN
Solar Energy Research Institute (SERI), Universiti Kebangsaan Malaysia
43600 UKM Bangi, Selangor,
MALAYSIA.
Email: [email protected] ; [email protected]
Abstract: Two modeling of solar desiccant cooling system in different modes of ventilation and recirculation
were designed based on hot and humid weather of Malaysia. The latent load and sensible load of the test room
were 12283.71 BTU/hr, 37533.56 BTU/hr respectively. The mass flow rates of process air and regeneration air
were 0.86 kg/s. The effectiveness of components is selected based on high efficent system because the main
objective of this study is comparison between ventilation and recirculation modes. By investigation of
simulation results of ventilation and recirculation modes, it was found that, amount of ventilation and
recirculation COP were 0.8, and 1.6 respectively. Therefore it was achieved that the recirculation solar
desiccant cooling system in hot and humid area is higher efficent than the ventilation solar desiccant cooling
system.
.
Keywords: desiccant cooling, ventilation, recirculation, solar energy
1. Introduction
Due to high electrical consumption of
conventional vapor compression systems, solar
desiccant cooling system is one of the promising
alternatives to cooling air where sensible and latent
heats of air are being removed separately [1]. The
first desiccant cooling system was recorded by
Pennington in 1955 [2]. Generally, depending on
using dehumidification material, there is two kind
of desiccant cooling system: solid and liquid. Many
Scientifics and researchers have studied in both
kinds of desiccant cooling by using renewable
energy [3]. The common materials used in solid
and liquid desiccant cooling system were selii-
cagel, and, liquid water-lithium chloride. Desiccant
cooling system is including of three main units
such as desiccant unit, heat source unit, and cooling
unit [4]. Depending on weather conditions of each
kind of weather data, elements of each unit become
designed. The components of basic cycle were
consisted of solid desiccant wheel as dehumidifier,
evaporative cooler as cooling unit, heat recovery
wheel as per-heater and per-cooler in regeneration
and process air side, respectively.
In recent years, according to Pennington cycle,
the different types of solid desiccant cooling cycles
were designed by various researchers on the world.
COP of desiccant cooling systems, regeneration
temperature, mass flow rate, fresh air, relative
humidity of supply air are the important parameters
which were considered by researchers. Jain et al.
[5] evaluated ventilation, and recirculation cycles
based on India weather data, to find effect of the
effectiveness of evaporative coolers on COP of
cooling system. Haddad, K et al [6] have studied
about simulation of a desiccant-evaporative cooling
system for residential buildings. They found that
the use of solar energy for regeneration of the
desiccant wheel can provide a significant portion of
the auxiliary thermal energy needed. Fong et al.[7]
have designed a simulation model (TRNSYS) of an
integrated radiant cooling by absorption
refrigeration and desiccant dehumidification.
Dezfouli M.M.S. et.al [8] investigated solar hybrid
desiccant cooling System in Hot and Humid
Weather of Malaysia. They found that solar hybrid
solid desiccant cooling system provided
considerable energy savings in comparison with
conventional vapor compression in hot and humid
area.
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This paper presents a comparison simulation
study on ventilation and recirculation of solar
desiccant cooling system in hot humid weather of
Malaysia.
2. Methodology
2.1 case study description
One solar desiccant cooling system with two modes
(ventilation and recirculation) was considered to
provide supply air for one test room in technology
park (UKM) of Malaysia. The latent load and
sensible load of the test room were 12283.71
BTU/hr, 37533.56 BTU/hr respectively. The
cooling capacity was 14.6 Kw. According to
ASHRAE comfort condition, the Indoor condition
designs are consist of temperature at 25 C , relative
humidity at 50% , and humidity ratio at 0.0098
kg/kg. According to Malaysia whether, the Outdoor
condition design are temperature at 35 C, and
relative humidity at 85%.
The cooling system was including four main parts
such as: 1- desiccant wheel as dehumidifier, 2-
Heat recovery wheel, 3- evaporative cooling as
humidifier, and 4- solar evacuated tube collector as
heat source. Simulation designing with the
TRNSYS software was carried out to investigation
of solar desiccant cooling in different modes to find
best operation of components and cooling system
based on hot and humid weather of Malaysia.
In this study, two modes such as ventilation mode
and recirculation mode were simulated with same
conditions and same amount of effectiveness of
components. Solar desiccant cooling effectiveness
of components is selected based on high efficent
system because the main objective of this study is
comparison between ventilation and recirculation
modes. Effectiveness of dehumidifier is including
two parameters F1 and F2 that were 0.05, and 0.95
respectively. The mass flow rates of process air and
regeneration air are 0.86 kg/s. The effectiveness of
heat recovery wheel and was 1. The saturation
efficiency of evaporative cooling was 1. Also,
efficiency of others components such as pump, fan,
heat exchanger was 1. Solar desiccant cooling
system description is divided to two parts that
explained in section 2.2, and 2.3.
2.2 ventilation mode of solar desiccant
cooling system
As shown in figure 1, ventilation mode of solar
desiccant cooling is open cycle that provides
supply air to room from ambient air. The return air
from room after few processes, releases to ambient
as exhaust air. So, there are two sides of air,
process side that produces supply air, and
regeneration side that releases return air from room
to ambient. In the process side, at the first step,
ambient air becomes dry by desiccant wheel. Then
heat recovery wheel acts as per cooling in process
air side. In the next step, air become cold by
evaporative cooler and then air goes to room as
supply air.
Figure 1: schematic of solar desiccant cooling in
ventilation mode
In the regeneration side, return air that taken latent
and sensible load from room, becomes cold by
evaporative cooler. Heat recovery wheel acts as per
heater in regeneration air side. Then heat from heat
exchanger and heater transfers to air. So, in the last
step of regeneration side, hot air takes humidity of
desiccant wheel and releases to ambient as exhaust
air. Figure 2 shows modeling of solar desiccant
cooling system in ventilation mode that was
designed by TRNSYS software.
Figure 2: Studio TRNSYS simulation for solar
ventilation desiccant cooling system
2.3 recirculation mode of solar
desiccant cooling system Figure 3 shows recirculation solar desiccant
cooling system. Generally, this system is not open
cycle. Process air side in recirculation mode is
close loop while regeneration air side is one open
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cycle. In process air side, room air that including
sensible load and latent load goes to desiccant
wheel to removing latent load. Heat recovery wheel
acts as per cooling in process air side. In the next
step, air become cold by evaporative cooler and
then air goes to room as supply air.
Figure 3: schematic of solar desiccant cooling in
recirculation mode
In the regeneration side, ambient air becomes cold
by evaporative cooler. In the next step, Heat
recovery wheel acts as per heater in regeneration
air side. Then heat from heat exchanger and heater
transfers to air. So, in the last step of regeneration
side, hot air takes humidity of desiccant wheel and
releases to ambient as exhaust air. Figure 4 shows
simulation modeling of recirculation mode solar
desiccant cooling system that was designed by
TRNSYS software.
Figure 4: Studio TRNSYS simulation for solar
recirculation desiccant cooling system
In this simulation, type 683 is desiccant wheel, type
506c is evaporative cooler, and type 760b is heat
recovery wheel.The type 91 is heat exchanger, type
71 is evacuated tube solar collector, type 3b is
pump, type 112a is fan, type 690 is zone load
(room), and type 109-TMY2 is weather data of
Kuala Lumpur (Malaysia).
2.4 determination of COP desiccant
cooling system
The Coefficient of Performance (COP) of the solar
desiccant cooling system can be calculated by rate
of heat extracted share on rate of heat regeneration.
Rate of heat extracted is cooling capacity of this
system that supplied cooling air to room. Rate of
heat regeneration is consisting regeneration heat
input by heater and solar thermal. Therefore, the
COP of the system is obtained by following
relation[9] (1):
generation
COOL
Q
QCOP
Re
=
According to figure 1, the COP of ventilation mode
can be written as:
)(
)(
79
45
hhm
hhmCOP
r
s
−
−=
According to figure 3, the COP of recirculation
mode can be written as:
)(
)(
810
45
hhm
hhmCOP
r
s
−
−=
Where ms (g/s) is mass flow rate of supply air, mr
(g/s) is mass flow rate of regeneration air, and
h(J/g) is enthalpy of air.
3. Results and Discussion
In this section, results of ventilation and
recirculation of solar desiccant cooling system are
explained and then compared. These results are
including temperature and humidity ratio of
ventilation and recirculation modes against of time.
Figure 5 shows important temperatures (OC) of
ventilation solar desiccant cooling system versus
time (h).
Figure 5: temperatures of different components in
ventilation desiccant cooling system
Temperature of ambient air, process air after
desiccant wheel, supply air, room, and regeneration
temperature are shown in this figure. Regeneration
temperature is one of the important temperatures
that has main role in changes of COP desiccant
cooling system. Regeneration temperature of
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ventilation mode is 70 oC almost. Figure 6 shows
important temperatures (OC) of recirculation solar
desiccant cooling system versus time (h).
Temperature of ambient air, process air after
desiccant wheel, supply air, room, and regeneration
temperature are shown in this figure.
Regeneration temperature of recirculation mode is
50 oC almost.
Figure 6: temperatures of different components in
recirculation desiccant cooling system
By comparison temperature results of ventilation
and recirculation, it can be considerable that
regeneration temperature of ventilation mode is
higher than the regeneration temperature of
recirculation mode. Temperatures of supply air for
both modes are almost same. Also temperatures of
the room for both modes are almost same.
Therefore, based one equation 1, the COP of
desiccant cooling system in ventilation mode is less
than COP of desiccant cooling system in
recirculation mode. Figure 7 shows humidity ratio
of cooling system components in ventilation mode
versus time.
Figure 7: humidity ratio of different components in
ventilation desiccant cooling system
Humidity ratio of ambient air, process air after
desiccant wheel, room, and supply air are shown in
this graph. Humidity ratio of supply air is one of
the important parameters that has main role in
amount of removing latent load from room by
desiccant cooling system especially in hot and
humid weather same as Malaysia. The amount of
humidity ratio of supply air in ventilation mode is
0.0108 kg/kg, while amount of humidity ratio of
room is 0.0124kg/kg. Figure 8 shows humidity
ratio of cooling system components in recirculation
mode versus time. Humidity ration of ambient air,
process air after desiccant wheel, room, and supply
air are shown in this figure. The amount of
humidity ratio of supply air in recirculation mode is
0.011 kg/kg, while amount of humidity ratio of
room is 0.0128 kg/kg.
Figure 8: humidity ratio of different components in
recirculation desiccant cooling system
By comparison humidity ratio results of ventilation
and recirculation, it can be considerable that
humidity ratio of room and supply air in
recirculation mode is a little more than humidity
ratio of room and supply air in ventilation mode.
Humidity ratios of process air after desiccant wheel
in both modes are same (0.008 kg/kg). According
to temperature and humidity ratio results of
simulation models of solar desiccant cooling in
ventilation and recirculation modes, the air
properties such as enthalpy for all of important
points in both modes were detected that shown in
table 1 and table 2.
Table 1: air properties of solar desiccant cooling
system in ventilation mode Temperature
(oC)
Humidity
ratio (gr/kg)
Enthalpy
(kj/kg)
T4 = 15 10.08 h4 = 40.4
T5 = 28 12.4 h5 = 59.8
T7 = 49 15.2 h7 = 88.7
T9 = 72 15.2 h9 = 112.5
Table 2: air properties of solar desiccant cooling
system in recirculation mode Temperature
(oC)
Humidity
ratio (gr/kg)
Enthalpy
(kj/kg)
T4 = 17 11 h4 = 44.9
T5 = 29 12.8 h5 = 61.9
T8 = 42 20 H8 = 93.8
T10 = 52 20 H10 = 104.2
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The COPs of ventilation and recirculation modes
were calculated by results data and equation 1. The
amount of COP of ventilation and recirculation are
0.8, and 1.6 respectively.
4. Conclusion
This paper presents a comparison study between
two modes of solar desiccant cooling system in hot
and humid weather of Malaysia. Ventilation and
recirculation modes were simulated by TRNSYS
software. Temperature and humidity ratio of
different points of solar desiccant cooling system
were results of both simulation modes. Results
show that COP recirculation mode is 2 times more
than COP ventilation mode. Therefore, it can be
concluded that recirculation solar desiccant cooling
system is higher efficent than ventilation solar
desiccant cooling in hot and humid weather of
Malaysia.
Acknowledgements The authors would like to thank the Solar Energy
Research Institute (SERI), University Kebangsaan
Malaysia for providing the laboratory facilities and
technical support.
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